While designing complex electronic systems, it is convenient and sometimes necessary to provide an adaptive load for testing. An example of such a complex electronic system is described by Michael J. Kavaya, Robert T. Menzies and Uri P. Oppenheim in a paper titled "Optogalvanic Stabilization and Offset Turning of a Carbon Dioxide Waveguide Laser," published in the IEEE Journal of Quantum Electronics, Vol. QE-18, No. 1, January, 1982 at pages 19-21. That paper describes a feedback loop employing the change in the discharge impedance of a CO.sub.2 laser, as its output power is varied, to stabilize the frequency of the laser. A zero offset feature in the feedback loop allows continuous tuning of the stabilized frequency over a 300-400 MHz range.
There are, of course, many closed-loop stabilization techniques for stabilizing the output frequency of a laser, such as a spectrophone stabilized laser, with line center offset, disclosed in U.S. Pat. No. 4,434,490, filed Mar. 31, 1982. It should therefore be understood that the method and apparatus for simulating a transfer function of a load in testing complex systems disclosed herein is not limited to laser frequency control systems, although the system of that application will be used as an example.